1. Field of the Invention
The present invention relates to a zoom lens and an image projection apparatus including the zoom lens. For example, the present invention relates to a zoom lens having a long back focus, providing a high level of optical performance, and suitable for a projector producing a projected image.
2. Description of the Related Art
There have been proposed a variety of projectors (image projection apparatuses) using a display device, such as a liquid crystal display device, to project an image formed on the display device onto a screen surface.
A projection lens included in such a projector is desired to have such characteristics as follows.
In a typical color projector including three display devices, light from a white light source is separated by a color separation optical system into light beams of red, green, and blue colors, which are guided to the corresponding display devices. The light beams ejected from the display devices are synthesized by a color synthesis optical system and are incident on a projection lens. In this configuration, to provide a prism or the like for synthesizing the color light beams passed through the display devices, it is necessary to leave a space between the display devices and the projection lens. Therefore, it is desirable that the projection lens have a certain length of back focus.
If the angles of light beams incident on the color synthesis optical system from the display devices vary, the spectral transmittance of the color synthesis optical system varies accordingly. As a result, the brightness of each color in a projected image varies depending on the field angle and thus, the projected image is difficult to see. To reduce such an effect depending on the field angle, it is desirable that a pupil on the display device side (reduction conjugate side) be substantially at infinity, that is, so-called telecentricity be good.
When pictures (images) on the display devices for three different colors are synthesized and projected onto a screen, the perceived resolution of displayed characters or the like is reduced if, for example, they look double. To prevent this, it is necessary that pixels of the respective colors be superimposed on one another throughout the screen. It is thus desirable that color misregistration (lateral chromatic aberration) that occurs in the projection lens be effectively corrected across the visible light spectrum.
To prevent a projected image from being distorted and becoming difficult to see, it is desirable that distortion be effectively corrected.
As an example of a zoom lens for a projector, there is known a zoom lens including a first lens unit disposed closest to the enlargement conjugate side, having negative refractive power, and fixed during zooming; and a last lens unit disposed closest to the reduction conjugate side, having positive refractive power, and fixed during zooming.
As an example of the zoom lens of this type, there is known a zoom lens in which at least three zoom lens units movable during zooming are disposed between the first lens unit and the last lens unit (see, e.g., U.S. Pat. No. 6,545,817).
In the zoom lens disclosed in U.S. Pat. No. 6,545,817, the second and third lens units disposed in this order from the enlargement conjugate side perform a primary zoom function. In this zoom lens, image plane variation during zooming is corrected by moving the third lens unit and other lens units that follow. In this type of zoom lens, the third lens unit is a very important lens unit for providing a high level of optical performance throughout the zoom range. Therefore, the third lens unit has at least three lens elements to improve optical performance.
A zoom lens for a projector is desired to have a long back focus and be telecentric on the reduction conjugate side. Therefore, a lens unit of negative refractive power is disposed on the enlargement conjugate side and a lens unit of positive refractive power is disposed on the reduction conjugate side.
With the lens configuration described above, since the entire zoom lens becomes asymmetric, it is difficult to provide a high level of optical performance throughout the zoom range. For example, negative distortion often occurs at the wide-angle end, and it is very difficult to correct it.
In the zoom lens disclosed in U.S. Pat. No. 6,545,817, the third lens unit has a greater share of zoom ratio than those of the other lens units. Accordingly, the third lens unit has refractive power greater than that of the other lens units and causes frequent occurrence of aberration variation during zooming. Moreover, due to the great refractive power of the third lens unit, a large amount of negative distortion at the wide-angle end tends to remain.
In the above-described zoom lens for a projector, it is important to set an appropriate lens configuration so as to ensure a predetermined zoom ratio, effectively correct aberration variation during zooming, and provide a high level of optical performance throughout the zoom range.
For example, it is important to appropriately define the refractive power of each lens unit (the second and third lens units, in particular) and the lens configuration of the third lens unit. If the configuration of the second and third lens units is not appropriate, it is difficult to effectively correct distortion at the wide-angle end and provide a high level of optical performance throughout the zoom range.